1. Field of the Invention
The present invention relates to a partial discharge detection method which enables non-contact detection of insulation deterioration in electric equipment by non-contact detection of a partial discharge arising from electric equipment, such as power distribution equipment, by means of the electromagnetic waveform generated by the discharge.
2. Description of the Related Art
In electric equipment primarily receiving a high voltage, such as pole-type equipment (an insulator or a potential transformer) provided in a power distribution line, e.g., power distribution equipment (distribution hardware) or a motor, so-called partial discharge stems from insulation deterioration, thus resulting in a high-frequency, wide-range electromagnetic waveform (i.e., high-frequency noise).
The partial discharge of this type is synchronous with a system power supply and generally arises in the positive and negative electrodes of a system power supply, and a resultant frequency becomes twice that of the system power supply.
Because of this, the electromagnetic wave usually changes in magnitude at a frequency twice as high as that of the system power supply.
For this reason, the applicants of the present invention have already invented a partial discharge detection method (filed as Japanese Patent Application No. Hei.8-160893). In this method, there is extracted a component changing at a frequency twice that of a system power supply of a received electromagnetic waveform of a preset measurement frequency. A partial discharge arising from electric equipment having deteriorated insulation is detected from the magnitude of the component or from a ratio of the magnitude of the component to the magnitude of background noise.
In case of the partial discharge detection method as described above, a partial discharge is detected by extraction of a component of an electromagnetic waveform changing at a frequency twice that of system power supply based on a partial discharge. Accordingly, as a result of a change in the environment or discharging state of a discharging portion of electric equipment having deteriorated insulation, a partial discharge arises only in the positive or negative electrode of the system voltage. When the electromagnetic waveform generated by electric discharge changes in magnitude in synchronism with the system power supply, a partial discharge cannot be detected.
More specifically, in a case where the electromagnetic waveform generated by a partial discharge changes in magnitude at the frequency of the system power supply, if a component changing at the frequency is extracted as a component changing at twice the frequency, the magnitude of the extracted component (i.e., an effective value) becomes smaller than the value of the component extracted as changing at the frequency of the system power supply. Consequently, a partial discharge cannot be detected, thus causing detection failures.